1. Technical Field
The invention relates to a length-measuring device used in connection with the rolling-up of thin, relatively soft, sheet material, such as paper and foil of aluminum, plastics, etc. Thus, the invention will find useful applications in paper mills, in foil-making mills and by manufacturers of plastics foils.
2. Prior Art
The rolling up of thin sheet material into a roll is performed either with the aid of a center-driven rolling-up device or with the aid of a circumferentially driven device. The rolled-up material will subsequently be used for the manufacture of books, newspapers, etc., and for a wide variety of other products which employ rolled-up sheet material as the "raw material". From the point of view of subsequent use, it is very important that the roll during its production is evenly wound and has a compact structure. This compact winding can be achieved in several different ways, for example by using a control means which strives to maintain a constant tension in the sheet material which is independent of the diameter and rotational speed of the roll. Numerous more or less sophisticated devices exist for dealing with this problem.
Length measuring of the rolled-up material is, in principle, very simple to perform. Before being wound on the roll, the sheet material normally passes some kind of deflector or supporting roller. By connecting a pulse transducer to the deflector or supporting roller, a measure of the rolled-up length can be obtained in a simple manner provided the diameter of the deflector or supporting roller is known.
Rolled material is often ordered in the form of a roll containing a specified number of meters of the sheet material in question. Therefore, the rolling-up plant will usually include apparatus for setting the desired length of the material on the roll. In such plant, a value obtained by means of the length-measuring device is compared continuously with the desired length, and when a certain material length remains to be wound on the roll, a retardation process in the rolling-up machine is started so that, as the correct wound-on length is achieved, the rolling-up operation can be stopped. Since the moment of inertia of the roll may be considerable, a retardation to practically zero speed on the approach to a measured, correct material length, is necessary.
Despite the provision of a variety of different control devices for controlling the wind-up process, ruptures of the material web arise more frequently than is desirable. The rolling-up machinery must then be stopped, loosely wound material on the roll must be removed, and the separated web parts must be rejoined. This operation affects the accuracy of length measuring, and to obtain an approximate agreement between the desired, set length and the actual rolled-up length, an estimate of the length of the removed material and of the length used for the join has to be made.
Now, in order to obtain the correct total length of the roll in the case of a joined material web, the previously set value of the desired length is normally increased by the estimated loss of material. Alternatively, the measured value of the rolled-up length may be reduced by the same estimated material loss.
Especially in printing works, printing presses, and the like, it is very important that the true length of the rolled-up material agrees with the stated length. The accuracy of the estimated material loss during the joining of the material web is of necessity not particularly great. There is therefore always a risk that, in rolls where a web break has occurred, the estimate of material lost will be too great and the finished roll will then contain more than the set length.
A direct measuring of the length of rolled-up sheet material on a roll, which has been subjected to a web break and rejoining during the rolling-up operation, has hitherto not been available. It has therefore been necessary to live with a certain uncertainty as regards the real length which is caused by the need to estimate the material lost.